SpringBoot + Hibernate JPA Lazy fetching mode still queries lists in an entity

Question

I am doing a REST API using springboot and JPA. I am trying to lazy fetching an entity in a One to Many relationship. Teacher to courses. I can see the sql statements done by JPA as I have the debuging option on.

In the controller, when calling a path all works great, but I can see that JPA is executing two queries. One for the teacher and another one for its courses. As I know, the lazy loading does not query until the data is required and I am not requiring it.

I have checked and conirmed that in the controller, when I retrieve the teacher data JPA does not query for the courses, but AFTER the return statement of the controller, somewhere, the courses are required and it loads everything when I call the teacher info from postman with a GET call.

It seems as if the LAZY loading is working correctly, but after the controller JPA loads the course list. If I do the EAGER fetching everything is loaded before the return statemnt.

I am not writing any code as I guess the question is more theorical than practical.

Does anyone know how this works?

Thank you so much!!!!

EDIT:

Teacher table

@Entity
@Table(name="profesores")
public class Profesor implements Serializable{

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

    @Column(name="nombre")
    private String nombre;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "profesor_id", referencedColumnName = "id")
    private List<Curso> cursos = new ArrayList<>();

}

Course Table

@Entity
@Table(name = "curso")
public class Curso implements Serializable {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long curso_id;

    private String nombre;

    @Column(name="profesor_id")
    private Long profesorId;
}

Controller

    @GetMapping("/profesor/{id}")
    public ResponseEntity<?> getProfesor(@PathVariable(value = "id") Long id){
        Profesor p = profesorService.findById(id);
        if(p!=null) {
            ResponseEntity<?> re = new ResponseEntity<>(p, HttpStatus.OK);
            //Just one query executed. I don't know the courses yet
            return re;
        }
        else {
            return new ResponseEntity<Void>(HttpStatus.NOT_FOUND);
        }
    }

After the return re; statement, somewhere, the courses are retrieved and JPA queries for them. I don't know what does the controller call, as I do directly from PostMan.

Answer 1

After returned Entity Profesor is serialized for response when serializer try to access courses to serialized for response then JPA load courses also. To solve this issue, You can create a response class for response (without courses field)

public class ProfesorResponse {
    private Long id;
    private String number;
    ...constructor
}

then map your entity in response object and return it.

Profesor p = profesorService.findById(id);
ProfesorResponse response = new ProfesorResponse(p.getId(), p.getNumber());